Introduction to Astigmatism and Corneal Irregularities

Abstract

A good knowledge of the geometry of the human eye in general and the cornea, is important for customized laser vision correction (CLVC). The difference between optical, visual, pupillary, and achromatic axes, in addition to line of sight, angles kappa, alpha and lambda, is important for understanding the basics of CLVC. The same can be said about corneal dimensions, zones, shape and power.

CLVC aims at improving both quality and quantity of vision by correcting the lower order aberrations (refractive errors) and the higher order aberrations (HOAs). The HOAs are induced by irregularity and asymmetry in the optical system of the eye. To understand the HOAs and their role in the management, definitions, classifications, and etiology of astigmatism, particularly the irregular type, should be understood.

Irregular astigmatism is evaluated subjectively and objectively. The evaluation starts from suspicion and goes through subjective refraction before it ends with ancillary tests, the most important being corneal topography/tomography and aberrometry. The former is essential to confirm the diagnosis, study the tomographic patterns of corneal maps and define ectatic corneal diseases (ECDs).

Objective corneal dioptric power (ODP) is a new concept. It measures the potential power of the cornea in reference to an average K reading of the normal population. This concept is based on understanding the factors affecting corneal power measurement and the types of corneal power maps. Calculating the ODP helps in understanding how the laser ablation profile works.